Spectroscopy in the terahertz (THz) frequency range has the potential to provide highly specific identification of molecular species in gas sensing applications. To take full advantage of what THz spectroscopy can offer, resolution on the order of megahertz (MHz) combined with the ability to tune by at least 1 THz is needed. One of the best ways currently to generate widely tunable THz radiation with the requisite resolution is to illuminate an electrically biased photomixer with two visible or near-IR lasers that operate at slightly different and tunable frequencies and generate a THz signal at the difference frequency. This has the advantage of using commercially available laser systems and operates at room temperature. Unfortunately, existing photomixers generate relatively small amounts of power (typically less than 1 microwatt).
The simplest way to detect a swept frequency source is to use a broadband direct detector that is sensitive only to the power incident on it. Unfortunately, in the THz frequency range direct detectors operating at room temperature are not very sensitive. Cryogenic cooling can increase the sensitivity, but limit the usefulness. Even with higher sensitivity, direct detection is still a problem with a broadband detector, since the power of the background radiation of objects at room temperature integrated over the frequency bandwidth of the detector can easily exceed the low power from the sources. Using an optical filter to limit the bandwidth is not beneficial unless the filter transmission is incredibly narrow and can be tuned to always peak at the THz frequency. At the moment such a filter does not exist.
One technique that has been used to increase the signal-to-noise employed (and is used in existing commercial systems) is to employ coherent homodyne detection. In this case, a second unbiased photomixer illuminated by the same two lasers will produce a finite DC current when also illuminated by the THz beam generated by the other photomixer. The direct current generated by other frequencies averages to zero (at least for frequencies that are further away from the desired frequency by more than the inverse of the measurement time). Since both detectors are illuminated by the same two laser sources, the detector spectral response is always centered on the source frequency. In this way, detection of the background radiation at other frequencies is significantly reduced, while sensitivity and speed are enhanced.
However, homodyne detection has a different problem. The detecting element does not respond to the intensity of the emitted beam, but the electric field instead. Because of this, the magnitude of the current depends on the relative phase of the THz signal reaching the detector and the phase of the beat frequency of the two visible lasers incident on the detector. This phase depends on the distance (measured in wavelengths) separating the source and detector. Therefore as the frequency is tuned the detector signal will vary even if the output power, the physical separation between the source and detector, and the transmission through the intervening media are constant. While this can of course be calibrated out, the signal-to-noise will vary with frequency, and at certain frequencies spectra will not be obtained. To compensate for this, data from multiple scans can be collected where each scan uses a different source/detector separation. However, this increases the measurement time and requires stable targets. In addition, by detecting a response at DC frequencies, 1/f type flicker noise adds significant noise.
Therefore, a need remains for a heterodyne photomixer spectrometer that enables room temperature, high resolution, and high speed detection of both amplitude and phase in a small package.